Hypereosinophilic conditions have been studied extensively, and we have identified new markers that distinguish among several important variants of this syndrome. The interaction between helminth infeciton and allergic disease has been studied using epidemiological tools coupled with physiological measurements of allergic disease and immunological assessments. We have demonstrated that chronic helminth infection protects against allergic diseases. Studies done to date have demonstrated the CD4+ cells (and neither the monocytes nor the basophils) are the overwhelmingly major source of IL-10 in chronic filarial infection. We have developed quantitative methods for examination of germline transcription for each of the antibody isotypes and have begun to use these methods to examine the molecular events that occur when antigens induce specific switch recombination events. Also baseline IgE and IgG4 germline transcription profiles have been measured in patients with allergic and parasitic infections and associated elevations of IgE and IgG4. An exhaustive search for parasite allergens that may cross-react with aeroallergens has been performed. We have identified a number of parasite antigens and the allergens orthologues and have studied the IgE and IgG responses to these. We have demonstrated unequivocally that parasite infections induced both parasite-specific IgE and IgE that interacts with enviromental allergens to which the parasite antigens are closely related.